The past fifteen years have seen an explosive growth of research interest in the study and application of organic materials as the active media in organic opto-electronic devices. This work has resulted in several advances, such as organic light emitting devices (OLEDs), including phosphorescent OLEDs. Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate.
Inorganic unipolar lasers are known, such as quantum cascade lasers (QCL). QCL differ fundamentally from the usual semiconductor heterostructure laser in that QCL devices are unipolar. In a unipolar quantum device, an electron or hole transition generating a photon is between sub-band energy levels within a same band (i.e., an intraband transition). Since the first experimental demonstration of an inorganic QCL device, Faist et al., Science 264, 553 (1994), progress has been made in understanding the physics and technological applications of such devices. Faist et al, Nature 387, 777; Gmachl et al., Nature 415, 883 (2002), Paiella et al., IEEE Photon. Tech. Lett. 12, 780 (2000). However, QCL may not enjoy many of the benefits of organic devices, such as low cost and tunability of energy levels at the molecular level.